This invention relates generally to switch-mode power converters. More particularly, this invention relates to methods and apparatus for constant power/current control for switch-mode power converters.
Switch-mode power converters typically include one or more semiconductor switches and energy storage elements, such as inductors and capacitors, and operate by switching the energy storage elements between various circuit configurations at a predetermined switching frequency. In a pulse-width modulated (“PWM”) converter, the output voltage or current of the power converter can be regulated by varying the duty cycle of one or more control signals applied to the switches.
Various analog and digital control methods have been used to provide line and load regulation of switch-mode power converters, such as DC-DC, AC-DC, DC-AC and AC-AC converters. For example, previously known control techniques for switch-mode power converters include constant power and current control.
To provide constant power and current control, two control loops are required: a voltage control loop and a current control loop. In a previously known constant power and current control system, sometimes referred to as a “voltage/current mode switching controller,” the voltage control loop provides voltage mode control in a constant voltage region of operation, the current control loop provides current mode control in constant power and constant current regions of operation, and control circuitry “ORs” the voltage control loop output and the current control loop output, and uses the smaller of the two outputs to generate a PWM signal.
For example, FIG. 1 illustrates the operation of a previously known constant power and current control system for a voltage/current mode switching controller having a nominal output voltage vnom, a nominal output current inom, a specified maximum output current imax, and a specified maximum output power Pmax. In this control system, for output current io values less than inom, voltage mode control is used. In particular, the voltage control loop compares the converter output voltage vo to a reference voltage, the error signal is provided to a compensator, and the compensator's output is used to set a switch duty ratio of the converter so that output voltage vo equals a desired value (e.g., vnom).
For output current io values greater than or equal to inom, control switches from voltage mode control to current mode control. In particular, for output current io values greater than or equal to inom, but less than imax, the current control loop compares the converter output current io to a reference current, the error signal is provided to a compensator, and the compensator's output is used to set a switch duty ratio of the converter. The current loop's reference current is set to (Pmax/vo) so that the converter output power, Po=(vo*io), is constant (e.g., Pmax). For higher loads, the current control loop clamps the output current at a constant value imax.
One problem with such previously known constant power and current control systems is loop saturation and output voltage overshoot. In particular, as the load increases, the duty cycle specified by the voltage control loop increases so that the converter can maintain the output voltage at vo=vnom, while supplying the required output current io. In such cases, for output current io greater than inom, the voltage control loop may saturate (e.g., the specified duty cycle will be maximum) when control switches from voltage mode control to current mode control. To avoid such loop saturation, some previously known control systems include anti-saturation techniques, such as clamping the integrator.
Whether or not anti-saturation techniques are used, however, if the load suddenly decreases, control will switch back from current mode control to voltage mode control. As a result, if the voltage control loop is saturated (or if anti-saturation techniques set the voltage reference to a fixed value corresponding to vnom), output voltage vo may exceed vnom. Depending on load conditions, the output voltage overshoot may be unacceptably large, and may damage sensitive electronic circuitry. Although not depicted in FIG. 1, previously known average current control techniques have similar output voltage overshoot problems.
Accordingly, improved methods and apparatus for constant power and current control for switch-mode power converters are desirable.